Optical amplifier

ABSTRACT

For setting the optical gain of an optical amplifier such as a Raman amplifier that is connected in a wavelength division multiplexing (WDM) system, the gain of the amplifier is made dependent on the states of optical polarizers connected to individual inputs of a WDM multiplexer. The polarizers can be actively controlled by a device connected to sense the output power of the Raman fiber at different wavelengths. For an appropriate control the optical gain can be given any desired shape such as for example a reasonable flatness. The control of the polarization states of the WDM-channels allows for the use of a single wavelength pump source of the amplifier, instead of the conventionally used multiwavelength source.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and a device forsetting, in particular equalizing or flattening, the frequency dependentgain due to polarization shifts in an optical amplifier, such as a Ramanoptical amplifier, used in a WDM system.

BACKGROUND

[0002] In recent years, the increasing demand for information capacityof optical fiber systems has made telecommunications manufacturersdevelop methods and devices for in particular wavelength divisionmultiplexing (WDM). For these systems, the signal information istransmitted on distinct channels of optical light. The signalinformation can comprise a plurality of logical signal channels, andeach signal channel may, in turn, include both time division multiplexed(TDM) and space division multiplexed (SDM) components, space divisionmultiplexing (SDM) meaning that separate fibers are used for differentparts of a message transferred in a logical channel.

[0003] The preferred wavelengths for most telecommunication opticalfiber systems are in the infrared part of the spectrum, around 1500 nm,due mostly to the low attenuation and the low signal pulse broadeningwhen transmitting signals on optical fibers in this region, but alsobecause of the availability of suitable light sources and detectors. Inparticular for WDM, another advantage here is the availability ofvarious types of optical amplifiers. These are necessary since eachwavelength channel carries only a small portion of the total power oflight propagating in the fiber and thus needs to be amplified tocompensate for optical losses in the fiber link, in order to get asufficient signal-to-noise ratio at the receiver end.

[0004] There are various designs of optical amplifiers. The mostimportant ones for telecommunication applications include erbium-dopedfiber amplifiers (EDFA), semiconductor optical amplifiers (SOA), Ramanamplifiers (RA), and optical parametric amplifiers (OPA). Theseamplifiers have specific advantages and disadvantages.

[0005] Raman amplifiers are of a special interest due to some importantfeatures. Such amplifiers differ from the others mentioned above in thatthe gain thereof is distributed over a given length of the optical fiberused, the Raman fiber. The Raman fiber is connected in series with theordinary transmission fiber, preferably near the transmitting lightsource. The power necessary for the amplification is delivered bypumping light from at least one separate pump light source. The maximumvalue and the shape of the Raman gain depend on the wavelength of thelight emitted by the pump light source, rather than on the fiber itself.Usually, injection of pump power takes place near the input end of theRaman fiber, using, e.g., a fiber-optical coupling device. Pump light ofdifferent wavelengths from several distinct pump light sources can beinjected in parallel in order to achieve a desired shape of the Ramangain, see the published International patent application WO 00/49721. Aproblem with this pumping method is that nonlinear interaction may takeplace between the various wavelength contributions. Also, the need forseveral pump light source and the intricate control thereof make suchamplifiers complicated and costly.

SUMMARY OF THE INVENTION

[0006] It is an object of the present invention to provide a method ofsetting, in particular flattening, the gain of an optical amplifier suchas a Raman amplifier and particularly to provide a reduction of thewavelength dependency of an optical amplifier used in a WDM system.

[0007] The above object is achieved by controlling, in a suitable waythe optical polarization states of the various channels at the input ofa WDM system to give a desired gain curve. This allows for using asingle pump source providing light of only one wavelength, instead of amultitude of pump light sources providing light of different wavelengthsthat is controlled as to its power, or a multiwavelength pump source, inwhich the light of each wavelength is controlled individually as to itsamplitude. The use of a single wavelength pump source is alsoadvantageous, because many different pump wavelengths may createnon-linear interaction between the pump contributions. The method ofcontrolling the input polarization states also makes the opticalamplifier that thereby obtains the desired gain robust and relativelyuncomplicated. Due to the fact that only a single pump light source isrequired, the amplifier has also a relatively small cost.

[0008] Using the control of the input polarization states the gain canbe controlled to have any predetermined shape within two maximum andminimum shapes. In this way, e.g. the gain tilt due to polarizationdependent losses may be compensated for over the whole optical link inwhich the optical amplifier is connected.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Embodiments of the present invention will now be described by wayof example, with reference to the accompanying drawings, in which:

[0010]FIG. 1 is a schematic picture of a Raman amplifier,

[0011]FIG. 2 is a diagram showing the maximum (solid line) and minimum(dashed line) Raman gain profiles around a wavelength of 1555 nm,

[0012]FIG. 3 is a diagram showing the flattening of the Raman gain overa bandwidth of 32 nm, and

[0013]FIG. 4 is a diagram showing the actively controllable change ofRaman gain within the maximum and the minimum gains.

DETAILED DESCRIPTION

[0014] In the following description, a Raman amplifier is used as atypical example of an amplifier for which the method can be used. Forother amplifiers having a similar behaviour comprising a gain dependenton the polarization states of the different amplified channels, the samemethod can obviously also be used.

[0015]FIG. 1 is a schematic diagram showing an actively controlled Ramanamplifier 1 connected at the input side of a WDM system. A plurality ofinput fibers 2, each carrying light signals of an individual wavelengthchannel, are connected to the input terminal of a WDM multiplexer (MUX)3. The light of each wavelength channel entering the multiplexer 3 iscontrolled as to its optical polarization state by a polarizationcontrolling unit 4. The output terminal of the WDM MUX 3 is connected tothe Raman fiber 5. Two optical couplers are connected in the Raman fiber5, one 6 a near the input end 7 a thereof and one 6 b near the outputend 7 b thereof. Typically, such couplers may consist of two fibersfused together. To one of the input terminals 9 a of the input endcoupler 6 a is an optical pump source 8 connected, injecting singlewavelength light. A multiple wavelength pump source is not neededbecause the corresponding effect for the system as a whole is achievedby using the plurality of polarizers 4, as will be describedhereinafter. At the output end coupler 6 b one of its output terminals 9b is connected to a channel power monitoring device 10 consisting of anarray of optical sensor elements, each sensor element measuring thepower of a specific channel wavelength, the power received in eachelement being converted to a corresponding electrical signal. Afteranalog/digital conversion each signal is further processed by anelectronic control unit 11 providing control signals fed back to controleach of the elements of the array obtained in the cases where thepumping signals having orthogonal and parallel polarizationsrespectively in relation to the polarization of the light of polarizers4.

[0016] A method of controlling the polarizers 4 in order to achieve apredetermined gain curve such as a flattening of the gain obtained atthe output end of the Raman fiber will now be illustrated by means ofthe exemplary diagrams of FIGS. 2-4. The dashed curve of FIG. 2 thusshows the minimum gain and the solid curve shows the maximum gain forlight propagating through a Raman fiber and amplified by light from apump light source as a function of the wavelength of the amplified lightin a typical case for a wavelength band located about a centerwavelength of 1555 nm. The minimum and the maximum gains are beingamplified. In a real case the gain will be somewhere in between thesecurves due to statistically varying properties of the Raman fiber. Thusit can be generally seen that the gain as measured at the output end 7 bof the Raman fiber depends on the wavelength of the amplified light.Also, the gain depends on the power and polarization state of the inputlight that is amplified in the Raman fiber.

[0017] In the diagram of FIG. 3 a most favorable value of flattened gainin a Raman fiber is illustrated by the horisontal solid line, this valuebeing equal to the peak value of minimum gain curve. For this gain valuea maximum flattened bandwidth of 32 nm could be achieved. This case canbe obtained by an individual, appropriate control of the channelpolarizers 4.

[0018] An extension of the flattening control concept may be carriedout, as illustrated by FIG. 4. The thick middle line having an irregularshape here illustrates some desirable shape of the gain in the Ramanfiber and is located between the maximum and the minimum gain curves. Byan appropriate individual control of the channel polarizers 4 any shapeof the gain as function of the wavelength can be actually obtainedwithin the constraints. In particular, this includes flattened gainshapes having a higher gain but having smaller bandwidths than thatillustrated in FIG. 3. Another possibility is the compensation of gaintilts due to wavelength dependent polarization losses over e.g. theoptical link connected to the output end 7 b of the Raman fiber.Furthermore, in combination with chromatic dispersion compensation infibers of DCF type the method described herein of adapting the gain in aRaman amplifier with wavelength may be very useful.

[0019] A general control scheme executed by the control unit 11 can beas follows. The control unit 11 sends control signals to the polarizers4 for adjusting the polarization of the light in the channels. Thesignals output from the elements of the optical sensor 10 representingthe power in the channels are compared to the desired gain in thechannels, while adjusting the corresponding elements of the array ofpolarizers 4 in small increments. When the desired gain has been reachedfor a channel, the adjustment of the polarizer for this channel isstopped.

[0020] A control scheme executed by the control unit 11 for setting theflattened gain as illustrated by the solid line in FIG. 3 can be asfollows. The first task is to find a minimum curve similar to that shownin FIG. 3. Thus, the control unit 11 sends control signals to thepolarizers 4 for adjusting the polarization of the light in the channelsto obtain the minimum gain value for each channel, i.e. the minimumpower level of the channel for changing polarization states of therespective input signal. Hence, the signals output from the elements ofthe optical sensor 10 representing the power in the channels areevaluated and stored, while adjusting the corresponding elements of thearray of polarizers 4 in small increments. If the power increases whenrotating the polarization by one increment in one direction, in the nexttrial a control signal having a value is produced rotating thepolarization by the same step but in the opposite direction. On theother hand, if the power decreases, the rotation direction when changingthe polarization state is maintained. This procedure is repeated foreach channel until a state is achieved in which an adjustment of thepolarization in either direction gives no further change or gives anincreased gain. The minimum value of the power is then represented bythe actual signal from the corresponding element of the channel sensor10. Thereupon the different stored values representing the minimum powerlevels for the amplified light of all WDM channels are evaluated and themaximum or peak value and the wavelength channel for which it isobtained are determined.

[0021] The next task is to adjust the gain in the WDM channels or morespecifically the power level, as observed at the output end of Ramanfiber 5, to the level of the determined peak value for as many channelsas possible which is the gain flattening procedure. Then, the storedvalues of the detected power levels can be evaluated again and for somechannels, the correct polarization state to achieve a gain equal to thedetermined peak value can be directly set as indicated by the storedvalues. For other channels, the adjustment method is continued, i.e. thesignal representing the optical power output from the respectiveelements of the optical sensor 10 is evaluated, again while adjustingthe corresponding polarizer elements 4 in small increments until theabsolute difference between the determined peak value and the read powerlevel reaches a minimum. If the absolute difference increases forrotating the polarization in one direction, the direction is changed forthe next rotary increment, and if the difference decreases, thedirection when changing the polarization state is maintained. Thisprocedure will continue until no further change in the absolute value ofthe power difference is observed or until the absolute values thereofincreases for rotation of the polarization state in either direction.

[0022] The method of applying individual polarizers 4 at each WDMchannel input in combination with using a single wavelength pump source8 has the equivalent effect on the Raman gain profile as by insteadusing a multiwavelength pump source, where each spectrum linecontribution is controlled as to its polarization and amplitude. Anadvantage of using a single wavelength pump source is that non-linearinteraction between different spectrum lines can be avoided.

[0023] As has already been mentioned and as should be obvious to anyoneskilled in the art, the method described herein comprising control ofthe polarization states of different wavelength channels input to anoptical amplifier can be used in any optical amplifier for which thegain of the optical amplifier for light of each of the wavelengthschannels are dependent on the optical polarization state of the light ofthe respective channel.

1. A method for setting the wavelength dependent gain in an optical amplifier for amplifying light of a plurality of WDM channels, the gain of the optical amplifier for light of each of the WDM channels being dependent on the optical polarization state of the light of the WDM channel, characterized by controlling (4) the optical polarization of individual ones of the WDM channels input (2) to the optical amplifier.
 2. A method according to claim 1, characterized in that the controlling of the optical polarization is made dependent on the optical power (10) in each of said individual ones of the WDM channels at the output of the optical amplifier.
 3. A method according to any of claims 1 and 2, characterized by the further steps of measuring values of the optical power in each of said individual ones of the WDM channels at the output of the optical amplifier and using the measured values for setting the optical polarization of the respective individual ones of the WDM channels input to the optical amplifier.
 4. A method according to claim 3, characterized in that the optical polarization of the WDM channels input to the optical amplifier are set to obtain a gain curve of the optical amplifier equal to a predetermined gain curve.
 5. A method according to claim 3, characterized in that the optical polarization of the WDM channels input to the optical amplifier are set to obtain a gain curve of the optical amplifier equal to a flat gain curve within a wavelength band including a plurality of WDM channels having adjacent wavelengths.
 6. A method according to claim 3, characterized in that the steps of measuring and using the measured values comprise the following substeps: changing the polarization of the different channels at the system input and evaluating the corresponding measured values, determining the minimum optical output power for each different channel, determining the peak value of the determined minimum output power for the channels, and setting the polarization of the channels to give measured output power levels deviating as little as possible from the peak value.
 7. A method according to claim 3, characterized in that the steps of measuring and using the measured values comprises changing incrementally or continuously the polarization of the different channels at the system input and evaluating the corresponding measured values and finally setting the polarization of the channels to give measured output power levels deviating as little as possible from desired power levels.
 8. A method according to any of claims 1-7, characterized in that the optical amplifier comprises a Raman amplifier including a Raman fiber (5).
 9. A method according to claim 8, in which the Raman amplifier includes a pump light source (8) injecting pump light into the Raman fiber (5), characterized in that pump light of a single wavelength is injected.
 10. An optical link comprising an optical amplifier for amplifying light of a plurality of WDM channels, the gain of the optical amplifier for light of each of the WDM channels being dependent on the optical polarization state of the light of the WDM channel, a power source connected to the optical amplifier to provide the power necessary for achieving the amplifying, characterized by an optical multiplexer (3) having a plurality of input light terminals for receiving light of different wavelength channels and combining the received light to a combined light signal issued to the optical amplifier, optical polarizers (4) connected to the input terminals for controlling the polarization state of the light of the wavelength channels, and a control unit (11) connected to the polarizers at the input terminals to control the gain of light of the wavelength channels amplified by the optical amplifier.
 11. An optical link according to claim 10, characterized in that the optical amplifier comprises an active fiber (5) and that the power source comprises an optical pump source (8) connected to inject pump light into the active fiber.
 12. An optical link according to claim 11, characterized in that the active fiber comprises a Raman fiber (5) to form a Raman amplifier.
 13. An optical link according to claim 11, characterized in that the optical pump source provides pump light of a single wavelength.
 14. An optical link according to any of claims 10-13, characterized by an array of optical sensors (10) connected at the output end of the optical amplifier measuring the optical output power of each wavelength channel and providing signals representing the measured output power to the control unit (11). 